The morning after the Fourth of July celebration, what did Grimes learn?

The morning after the Fourth of July celebrations, Grimes learns that Lucy has escaped from the plantation. As the overseer, it's Grimes's job to make sure that all the slaves are herded on and off the wagons at the beginning and end of each working day. But on this particular occasion, when everyone returns to the plantation, he notices that Lucy's nowhere to be found.
Caline tells Grimes that Lucy's been sick and so spent all day in her cabin recovering. Grimes is angry that Lucy appears to have stayed home without asking permission. He tells Bob to go to Lucy's cabin and check if she's still there. The guard thinks that she is, but it turns out the next morning that it was just a dummy's head lying on Lucy's pallet and that she's actually escaped. So Grimes puts together a posse of men and bloodhounds to go find the runaway slave.